1. Field of the Invention
This invention relates to a process for the preparation of polymethylene terephthalate of high viscosity. More specially, this invention relates to the preparation of high viscosity polymethylene terephthalate containing antimony oxide alone or in admixture with another additive, especially a fire retardant agent. This invention is further directed to the preparation of a high molecular weight (high viscosity) polymethylene terephthalate, which can be readily formed into final objects by injection molding and which has not undergone unnecessary thermal degradation processes.
2. Discussion of the Prior Art
Among the more recent thermoplastics, the thermoplastic polyesters have rapidly gained a large share of the market. Particularly polytetramethylene terephthalate (PTMT), on account of its easy working qualities and its good mechanical and electrical properties, has become a popular material, especially for fabrication by the injection molding process. For many purposes, however, especially in the electrical industry, but also in the automotive industry and in the manufacture of machine parts, a fire-retardant material is needed, and such material is also being offered on the market.
PTMT is treated for fire-retardancy almost exclusively with organic compounds such as brominated aromatics and others which are mixed with the polyester in the solid state and are incorporated and blended in as finely and uniformly as possible by melting and working in a kneader or extruding machine. Usually a strand is extruded from this fused mixture and is chopped into granules and marketed in this form. To improve the flame-retardant action of these brominated additives, so that the amount that is used can be kept within economically acceptable limits, it is common to incorporate into the PTMT together with the flame-retardant agents, synergistically acting additives such as antimony compounds, boron or zinc compounds and the like, although of these only antimony trioxide is of practical importance.
Although material of good flame-retardancy can be obtained by the above-described method, the process is nevertheless problematical for other reasons. The additives affect not only the combustibility of PTMT but also its other properties. In particular, they impair its mechanical characteristics, such as elongation at rupture, impact and notch impact strength, i.e., they produce brittleness. It is known that the mechanical characteristics of substances of high molecular weight such as PTMT depend on their molecular weight, which is measured on the basis of their solution viscosity. The higher the molecular weight and hence the solution viscosity is, the better are the mechanical properties; the lower the solution viscosity is, the more brittle and fragile the material becomes. Since the PTMT is made more brittle by the addition of fire-retardant agents and synergistic materials, it has hitherto been the practice to counter this undesirable effect by using a PTMT of the highest possible viscosity. This, however, is severely limited by practical and economical considerations. In the present state of the art, high viscosities are not achieved immediately in the production of the PTMT by polycondensation, but afterwards, by an additional procedure. In this procedure the granules of commonly low viscosity are cured for many hours at elevated temperature and reduced pressure, until at last the desired viscosity is achieved by this "solid-phase condensation", as it is called. In order to achieve a fire-retardant PTMT of useful mechanical properties, it is necessary to use as starting material a PTMT which has been cured to reduced viscosity (.eta..sub.red) values of approximately 1.6 dl/g or more, which involves a considerable expenditure of time and energy.